Great hopes are placed on light control devices using an SPD in the field of applications such as building materials aimed at energy saving through incident sunbeam control and display devices and optical devices using light transmission and reflection control functions. An SPD is a device whose optical characteristics vary between when no voltage is applied thereto and when a voltage is applied thereto. Generally, in an SPD, electrodes are formed on transparent substrates disposed to mutually face and a suspension containing light control particles (hereinafter also referred to simply as “particles”) which are dispersed therein and which react to an electric field is provided between the substrates and between the electrodes.
The light control particles to be filled in an SPD are those which show optical anisotropy for absorption and reaction. Such light control particles include those which are caused to show optical anisotropy by their anisotropic shapes. Hence, particles whose aspect ratio is not 1 and which are rod-shaped, needle-shaped or shaped like flat plates are used as light control particles.
When no voltage is applied, the light control particles in a suspension are engaged in Brownian motion to be oriented randomly, so that light incident on the suspension is absorbed and scattered to be blocked without being transmitted.
When a voltage is applied, on the other hand, the light control particles in a suspension are affected by dielectric polarization caused by an electric field and are uniformly oriented along the direction of the electric field. In this state, light incident on the suspension is modulated by the oriented light control particles. According to Japanese Patent Laid-Open No. Sho 53 (1978)-144893, for example, when the direction of incident light is the same as the direction of the electric field and the direction along which the particles are oriented, the incident light can be transmitted through the suspension to achieve high transmittance and such a state is referred to as an open state.
Another method for controlling a light control area in an SPD is disclosed in Japanese Patent Laid-Open No 2005-156811. In the method, plural electrode pairs are formed on a substrate or on a spacer disposed between substrates and control is effected by selecting an electrode pair across which a voltage is to be applied.
When controlling a light control area using prior-art SPD technology, however, it is necessary to select an electrode pair across which a voltage is to be applied out of plural electrodes. For light-control area control, therefore, it is necessary to perform passive matrix drive or active matrix drive. Furthermore, to realize more continuous light-control area control, it is necessary to increase the number of electrodes. Increasing the number of electrodes, terminals and matrix drive elements makes the SPD structure more complicated resulting in increasing the number of production processes to eventually increase the production cost.
Such a cost increase can be avoided by developing SPD technology which makes it possible to continuously control a light control area between electrodes. An object of the present invention is to provide technology for realizing light-control area control between electrodes by focusing attention on light control particle material, SPD structure, and SPD driving method.